THE EPIDERMAL TISSUE. 
97 
of the inner tissue; the walls sometimes show slender thickening-bands running in 
a spiral manner, and open externally by large orifices, being also in communication 
with one another by similar ones (/). In the mature state they contain nothing 
but air or water, which rises in them by capillarity. Within this epidermal tissue the 
stem is similar to that of Mosses; the cells become towards the surface gradually 
narrower, thicker-walled, and of a darker colour. A similar epidermal layer, and with 
similar hygroscopic properties, occurs in the aerial roots of Orchids and of some 
Aroideae. 
Like the other forms of tissue, the epidermis also attains a greater perfection in the 
sporogonia of Mosses; the variously differentiated internal tissue of the sporangium 
Fig. 82. — Part of a radial longitudinal section through the sporangium of Fuuarz'a hygro7netrica (X 300) ; e epidermis; the tliick 
black line on the outside is the cuticle. (For further explanation of the Fig. see Book II.) 
is surrounded by a highly developed true epidermis, sometimes provided with stomata 
(Fig. 82). 
(b) The IEpiderm.is^ In Vascular Plants the epidermal tissue consists usually only 
of a single superficial layer of cells, the true Epidermis. In its origin it always consists of 
a single layer ; but this sometimes sphts into two or more by divisions parallel to the 
^ H. von Mohl, Vermischte Schriften bot. Inhalts. Tübingen 1845, p. 260— F. Cohn, De 
Cuticula, Vratislavise 1850.— Leitgeb, Denkschriften der Wiener Akad. 1865, vol. XXIV. p, 253. — 
Nicolai, Schriften der phys.-ökonom. Gesells, Königsberg, 1865, p. 73. — Thomas, Jahrb. für wiss. 
Bot. vol. IV. p. 33. — Kraus, ibid., vol. IV. p. 305, and vol. V. p. 83. — Pfitzer, ibid., vol. VII. p. 561, 
and vol. VIII. p. 17.— De Bary, Bot. Zeitg. 1871, nos. 9-11 and 34-37. 
H 
